1. Field of the Invention
The present invention relates to a photometering apparatus for a camera having an interchangeable focusing plate.
2. Description of the Related Art
In general, a single lens reflex camera is provided with a view finder in which light carrying an object image formed by a photographing lens is converged onto a focusing plate (focusing screen) which is located in a position optically equivalent to a film surface, whereby the object image formed on the focusing plate can be enlarged and viewed as an erect image through a magnifier and an image erection system (pentagonal prism). To detect the brightness of the object, a photometering device (photometering sensor) is provided in the vicinity of the magnifier; the object image formed on the focusing plate is re-formed on a light receiving surface of the photometering sensor to measure the object brightness in accordance with the output of the photometering sensor corresponding to the object.
There are known various focusing plates including those which are provided on the center portion thereof with a split image or a microprism, or provided on the entire surface thereof with a mat.
In a camera in which a replaceable screen on which a desired focusing plate can be optionally mounted, the state, or the quantity of the object image light rays transmitted through the focusing plate varies depending on the characteristics of the mounted focusing plate. Therefore, for different focusing plates, the state, or quantity of the object light incident upon the photometering sensor is different, thus resulting in an unreliable measurement of the object brightness. This is shown in FIGS. 16 and 17 by way of example.
FIG. 16 shows an example in which, in a spot beam photometering operation, an exposure error caused by the different characteristics of the focusing plates varies as the minimum diaphragm value (open aperture) Avmin of the photographing lens changes. FIG. 17 shows an example in which an exposure error caused by the different characteristics of the focusing plates varies as the position of the exit pupil changes.